These curved trenches on Jupiter's icy moon Europa indicate that its poles have drifted almost 90 degrees from their original position, in a phenomenon called true polar wander, according to a new study. Researchers believe that Europa contains a hidden layer of liquid water resulting from internal heat generated by Jupiter's fluctuating gravitational tug on the moon (think of ocean tides here on Earth). If so, its crust ought to move relatively freely, disconnected from the rocky core. Writing in Nature, researchers say that the troughs, which extend for some 300 miles (500 kilometers), are consistent with stresses having rotated the crust by 80 degrees—just the kind of free movement you'd expect from a crust floating on a hidden ocean. They suggest that ice might have piled up at the poles, causing a wobble similar to that of a topheavy gyroscope.